The present invention relates to magnetic memory devices and more particularly to an improved bias field assembly for use in such devices.
Magnetic domains or bubbles are minute cylindrical areas that can be generated and maintained in thin films of magnetic material. These cylindrical areas, which are magnetized in the opposite direction from the rest of the thin film material, can be moved or propagated along Permalloy tracks on the film surface when subjected to a rotating magnetic field. Track arrangements for performing shift functions and logic operations are well known in the art.
Magnetic domains are maintained by a magnetic bias field produced by a bias field assembly which normally includes two permanent magnets having dissimilar magnetic charges and two magnetically permeable plates for distributing the magnetic charges. The magnetically permeable plates parallel the upper and lower surfaces of an assembly including substrates on which the magnetic domain films are mounted. These substrates are surrounded by a drive coil assembly which includes a pair of orthogonal coils which may be energized by high frequency alternating currents to establish a rotating magnetic field. It is this field which causes the magnetic domains to propagate along the Permalloy tracks on the film surface. Magnetic bias field apparatus of this kind is disclosed in U.S. Pat. No. 3,927,397, assigned to the assignee of the present application.
The rotating magnetic field induces eddy currents in the permeable plates above and below the drive coil/substrates assembly. The eddy currents increase the effective resistance of the drive coil, thereby increasing the power required to maintain a suitable magnetic field. Power losses create heat dissipation problems.
Power losses and heat dissipation requirements were not considered to be critical with prior art magnetic domain memory devices since such devices used smaller drive coils which operated at lower frequencies. Smaller drive coils operating at the lower frequencies did not induce unacceptable levels of eddy currents. However, magnetic domain memories have been increasing in size in order to increase memory capacities. Similarly, the frequencies of alternating currents used to energize drive field coils have been increased to raise propagation speeds and thus data transfer rates. Greater eddy currents and hysteresis losses have resulted leading, in turn, to severe power losses and heat dissipation requirements.